All responsible governments are greatly concerned about the possibility that the highly lethal H5N1 influenza A viruses that have been killing large numbers of wild and domestic birds (and a few people) in South East Asia may suddenly change their host range and begin to spread globally, with extreme rapidity, among human populations. Though we have an effective anti-influenza drug (Oseltamivir), a new-generation H5N1 vaccine is in an advanced stage of development, and the influenza virus research, surveillance and public health networks are first class, the capacity of these viruses to mutate very rapidly means that there is no way to be absolutely sure that we will not be hit by a virus against which we relatively little protection. The current formalin-inactivated vaccines promote a highly specific and effective antibody response, provided this is directed against exactly the right virus, but they do not stimulate the more cross-reactive cell-mediated immunity (CMI) that can give some measure of protection against a broad spectrum of influenza A viruses. The question is, should we be thinking about vaccine strategies that also induce virus-specific CD4+ and CD8+ "memory" T cells, in the knowledge that the more rapid emergence of these effectors of CMI will, at best, ameliorate the severity of the disease rather than prevent infection completely? Though it is the case that many people will already have some influenza-specific T cell memory, it is also likely that regular boosting will greatly increase both the numbers of these memory T cells and the rapidity of the recall response following live virus challenge. The present experiments address the effectiveness and characteristics of the CMI response in mice that have (like people) previously been exposed to heterologous influenza A viruses, then infected with these highly virulent H5N1 influenza A viruses. A variety of genetically engineered and reassortant viruses will be used in prime, boost and challenge experiments to investigate the limits of protection and the cellular mechanisms that might be selectively promoted if an effective CMI- directed vaccine were to be developed. Whether the H5N1 viruses have particular characteristics that tend to subvert the protection conferred by pre-existing CMI will also be addressed, in the hope of identifying strategies that might be used to defeat such effects. These studies should allow us to develop a clear, mechanistic understanding of how to promote effective CMI against the H5N1 viruses.